Most big tech companies have a constant stream of patent applications, many of which are granted. But they rarely sue over 99% of them. Sure, Amazon had one-click for however many years and they cared about it. But the other tens of thousands of patents Amazon owns (a couple of which have my name on them as 'inventor') are never going to see a court room.
Why do they patent everything? So that patent trolls can't patent it first and then sue them for patent violations. It's a defensive patent.
Is ECS going to be Unity's "This is ours, no one else can use it without paying us" lawsuit-generating patent? I doubt it. They know they'd lose in court eventually, and along the way the entire industry would stop doing business with them. It would be a complete lose-lose move.
On the other hand, now that Unity owns this patent, no patent troll can go and patent ECS then start suing small-studio Unity customers, like parasites leaching off the success of Unity.